SECTION VI 



THE NERVOUS SYSTEM 



ORIGINALLY WRITTEN n\ H. ST. JOHN BROOKS. M.I). REVISED FOR SECOND AND THIRD 



EDITIONS MY AIITIH H KOMINSON 



REVISED AND LARGELY REWRITTEN FOR THE FOURTH EDITION 

 BY IRVIXC IIAHDESTY, A.B., PH.D. 



ASSISTANT PROFESSOR OF ANATOMY, UNIVERSITY OF CALIFORNIA 



nervous system of man, both anatomically and functionally, is the most 

 highly developed and definitely distributed of all the systems of the body. It 

 consists of an aggregation of peculiarly differentiated tissue-elements, so arranged 

 that through them stimuli may be transmitted from and to all the other tissue systems. 

 It is a mechanism with parts so adjusted that stimuli affecting one tissue may be 

 conveyed, modified, and distributed to other tissues so that the appropriate reac- 

 tions result. While protoplasm will react without nerves, while muscle will con- 

 tract without the mediation of nerves, yet the nervous system is of the most vital 

 importance to the higher organisms in that the stimuli required for the functioning 

 of the organs are so distributed throughout their component elements that the 

 necessary harmonious and coordinate activities are produced. For this purpose 

 the nervous system ]>enneates every organ of the body, the nerves dividing into 

 smaller and smaller branches till the division attains the individual nerve-fibres of 

 which the nerves are composed, and even the fibres bifurcate repeatedly before their 

 final termination upon their allotted elements. So intimate and extensive is the dis- 

 tribution throughout that could all the other tissues of the body be dissolved away, 

 still there would be left in gossamer its form and proportions a phantom of the 

 body composed entirely of nerves. 



The parent portion or axis of the system extends along the dorsal mid-line of 

 the body, surrounded by bone and, in addition, protected and supported by a series 

 of especially constructed membranes or meninges, the outermost of which is the 

 strongest. The cephalic end of the axis, the encephalon, is remarkably enlarged in 

 man, and is enclosed within the largest portion of the bony cavity, the cranium, 

 while the remainder of the central axis, the spinal cord, continues through the for- 

 amen magnum and lies in the spinal canal. 



The intimate connection of the axis with all the parts of the body is attained 

 by means of forty-three pairs of nerves, which are attached to the axis at somewhat 

 regular intervals along its extent. They course from their segments of attachment 

 through the meninges and through their respective foramina in the bony cavity to 

 the periphery. Of these cerebro-spinal nerves twelve pairs are attached to the 

 encephalon, and thirty-one pairs to the spinal cord. Most of them contain both 

 afferent fibres, or fibres which convey impulses from the peripheral tissues to the 

 axis, and efferent fibres, or fibres which convey impulses from the axis to the 

 peripheral tissues. All the thirty-one pairs of spinal nerves possess fibres of both 

 types, though in varying proportions. 



I'pon approaching the spinal cord, each spinal nerve is separated into two roots 

 its dorsal root and its ventral root. The afferent fibres enter the axis by way of 

 the dorsal roots, which are. therefore, the sensory roots, and the efferent fibres leave 

 the axis by way of the ventral or motor roots. 



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